In recent years, there is developed a white LED having characteristics such as compactness, a longer operating life, low voltage drive, and being free of mercury, in relation to mercy gas excitation-type fluorescent light tube (FL) and cold cathode fluorescent light tube (CCFL), which have been used for general lighting and backlights for liquid crystal display devices.
With regard to the white LED, there are a type 1 in which white light is emitted by combining light emitting diodes of three colors of a blue-light light emitting diode, a green light emitting diode, and a red light emitting diode, and a type 2 in which white light is emitted by combining a long-wavelength ultraviolet-light (300 to 430 nm) or blue-wavelength light (460 to 480 nm) emitting diode as an excitation source and a phosphor layer containing a phosphor of a single color or a plurality of colors which emits visible light. Since the white LED of the type 2 leads to reduction of the number of light emitting diodes and heat generation of the entire can be suppressed, development of the white LED of the type 2 is vigorously done in recent years.
Among the white LEDs of the type 2, the one using the light emitting diode (hereinafter, referred to as an ultraviolet-light light emitting diode) emitting a long wavelength ultraviolet ray (300 to 430 nm) obtains white light by using phosphors of three colors of blue, green, and red as phosphors.
On the other hand, the one using the light emitting diode (hereinafter, referred to as a blue-light light emitting diode) of the blue wavelength (460 to 480 nm) obtains white light by using phosphors of two colors of green and red or a phosphor of yellow as a phosphor(s).
Among the white LEDs using the blue-light light emitting diode, the one using the phosphors of two colors of green and red has a problem that color unevenness is large and that dispersion of chromaticity is also large. The one using the yellow phosphor has a problem that color reproducibility is narrow, that color rendering is also bad, and that designing to a predetermined chromaticity is difficult. On the other hand, the white LED using the ultraviolet-light light emitting diode, where there is a wide variety of phosphors to be used compared with the one using the blue-light light emitting diode and thus color reproducibility is wide and predetermined chromaticity is easier to be obtained, is suitable for a backlight light source in a liquid crystal display device.
As the phosphor used along with the ultraviolet-light light emitting diode, for example, an europium-activated halophosphate phosphor or an europium-activated aluminate phosphor as a blue phosphor, a copper/aluminum-activated zinc sulfide phosphor or an europium/manganese-activated aluminate phosphor as a green phosphor, and an europium-activated yttrium oxysulfide phosphor or an europium-activated lanthanum oxysulfide phosphor as a red phosphor are combinedly used.
Meanwhile, the white LED using the ultraviolet-light light emitting diode is manufactured, for example, by adding and mixing phosphors of three colors of the blue phosphor, the green phosphor, and the red phosphor to a transparent resin thereby to prepare a phosphor slurry, thereafter applying this phosphor slurry onto the ultraviolet-light light emitting diode, and curing the phosphor slurry. A compounding amount of the phosphor, properly chosen according to aimed chromaticity, is that, for example, among a total amount of the phosphors of the blue phosphor, the green phosphor, and the red phosphor, the blue phosphor occupies equal to or more than 15 mass % and equal to or less than 25 mass %, the green phosphor occupies equal to or more than 15 mass % and equal to or less than 25 mass %, and the red phosphor occupies the rest (for example, see JP-A 2007-096133 (KOKAI)).
In recent years, usage of white LEDs as backlight light sources in a variety of liquid crystal display devices such as small screens of a cellular phone, a car navigation system, a mobile communication device and the like, and medium/large screens of a personal computer and TV and so on has been discussed.
However, with regard to the white LED using the ultraviolet-light light emitting diode, there is a problem that chromaticity gradually changes after light emission due to temperature rise by heat generation by itself. Concretely, values x, y in an XYZ color system of CIE change in a manner to gradually increase in accordance with temperature rise. Further, such change of chromaticity halts light emission of the white LED, and is repeated after light is emitted again.
As stated above, since chromaticity may change upon temperature rise in a white LED using an ultraviolet-light light emitting diode, application thereof to a backlight light source in a liquid crystal display device is not necessarily easy. In particular, since a cellular phone, a car navigation system, a mobile communication device and the like may be left in a sun-heated vehicle and reach quite a high temperature, a white LED as a back light source in that liquid crystal display device is required to have a small chromaticity change due to temperature rise.